This invention relates generally to the field of paper making and, more particularly, to methods and apparatus in a drying section of a paper machine.
More specifically, the present invention relates to apparatus in the drying cylinders of a paper machine for improving the heat transfer characteristics of the drying cylinders. The apparatus is situated on the inner surface of the cylindrical mantle of the drying cylinder to increase the turbulence of the condensate layer which forms on the inner surface during operation to thereby improve the heat transfer in the drying cylinder during the paper making operation. The apparatus comprises a plurality of turbulence ribs attachable to the inner surface of the mantle of a drying cylinder by means of attachment bands and tensioning means.
It is known that water is condensed on the inner surface of a steam heated drying cylinder in a drying section during operation of the paper machine. The water is removed by any one of a number of known suction arrangements and in this connection reference is made to Finnish Pat. No. 62,694 of Feb. 10, 1983, assigned to the assignee of the instant application.
The presence of condensate on the inner surface of the drying cylinder mantle inhibits the heat transfer from the steam within the cylinder to the outer surface thereof. In order to overcome this problem the provision of ribs on the inner surface of the drying cylinders has been suggested to convert the laminar flow of the condensate water into a turbulent flow to thereby improve the heat transfer through the condensate layer. In this connection ribs have been attached to the inner surface of the drying cylinder mantle in various ways. Additionally, ribs have been machined so as to extend integrally from the inner surface of the cylinder mantle in the axial or radial direction whereby heat transfer is improved within the region over which the ribs are effective. Furthermore, various types of nets or similar constructions have been used in addition to ribs to convert the laminar condensate flow to a turbulent flow.
Reference is made to the following patents which exemplify the prior art pertaining to the present invention:
SE Pat. Appln. No. 78-066,040 (Beloit Corp.), DE Pat. No. 2,330,199 (J. M. Voith GmbH), DE-AS No. 2,844,273 and DE-OS No. 2,846,414 (Escher Wyss GmbH), DE-AS No. 2,849,454, and DE-AS No. 2,903,784 (J. M. Voith GmbH), U.S. Pat. No. 3,724,094 (Kimberly-Clark Corp.), and U.S. Pat. No. 4,195,417 (Beloit Corp.).
The known arrangements for providing ribs on the inner surface of a drying cylinder mantle have several drawbacks. In particular, the construction of such arrangements is generally quite complicated and the cost of manufacture has generally been quite high. Moreover, the installation of known rib arrangements in position has been found quite difficult. The manufacture of machined or integral ribs as well as the installation of certain prior art rib arrangements in drying cylinders that are in use is generally not possible without having to detach and disassemble the drying cylinder which of course is undesirable from the standpoint of the output of the paper machine.
Another serious problem of known rib arrangements is encountered in attempting to obtain a reliable fastening or attachment of the rib arrangements to the inner surface of the drying cylinder mantle. This problem arises from the fact that the ribs and the means provided for attaching the ribs to the drying cylinder are installed within the drying cylinder while the latter is cold. It has been found that when the paper machine is run and hot steam supplied into the drying cylinders which of course significantly increases the temperature thereof, various parts of the drying cylinder and rib arrangements undergo thermal expansion of relatively different magnitudes. The turbulence ribs may obtain a curved or warped configuration due to such thermal expansion whereupon the ribs may become detached from the inner surface of the drying cylinder mantle. Such phenomena detracts from the proper functioning of the rib arrangement which in turn results in an uneven temperature profile over the outer surface of the drying cylinder.